The monitoring of fetal breathing movements may be of value as an index of fetal maturation and well-being. The establishment of what is a normal gestational pattern of respiratory activity would appear to be mandatory, if such a measurement is to become clinically useful. In this study, the diaphragmatic electromyogram (EMG) has been recorded, as an index of respiratory activity, in chronically instrumented 'in utero' fetal lambs over the last one-third of gestation. During this period an attempt has been made to define a normal, gestationally related pattern of respiratory activity. Seven fetal lambs of known gestational age were instrumented with recording electrodes and vascular catheters at approximately 95 days gestation. Following a recovery period of at least three days, regular recordings were commenced and continued throughout gestation and into the newborn period. Recordings were taken over a two-hour period, between 0830 and 1530 hours. Computerized data handling facilities were used and each discrete burst of EMG activity was defined and counted for each minute of the entire recording period. The rate of respiratory activity per minute, thus gained, varied markedly from minute to minute over each two-hour recording period. The pattern of variability of respiratory activity was seen to change with advancing gestational age. The total number of bursts of respiratory activity for each two-hour period was seen to be linearly correlated with gestation. As gestational age approached term, the total number of bursts of respiratory activity steadily declined. The time intervals between discrete bursts of BMG were taken as representing an expiratory time Te. Those expiratory times greater than 3 seconds and 10 seconds respectively, were taken and each group summed and expressed as a percentage of the total recording time. These percent Te's were seen to be linearly correlated with gestation. There being a progressively greater percentage of time where there are extended periods of no respiratory activity as gestation advances it is speculated that the gestationally related changes in patterns of respiratory activity described above, are related to the development and maturation of central neural control mechanisms.